A pump of the abovementioned type is known from SE-B-467 466 (corresponding to U.S. Pat. No. 5,039,320). In this pump, there are openings between the hub and the annular partition, through which the fibrous pulp suspension and the air or gas which is to be separated can pass from the front to the rear pump chamber part in a radial/axial direction, in which the axial movement component is considerable. This has the consequence that considerable axial forces arise in the pump casing, acting on the pump wheel, which in turn makes the mounting of the drive shaft of the pump wheel more difficult and calls for the use of special bearings. Another consequence is that the gas/air separated in the axial direction will contain an undesirable content of fibrous pulp.